Alkanediols such as butane-1,4-diol, are the focus of a lot of attention in view of their high potential for the chemical industry, especially in the preparation of polyurethane foams and polymers, in polyesters as well as for the preparation of tetrahydrofuran. In spite of a still growing demand for these alkanediols the use of these alkanediols is still rather restricted due to the problems encountered in their preparation, which generally proceeds via the catalytic hydrogenation of the corresponding alkynediol. With the preparation of e.g. butane-1,4-diol, it is known that the catalytic hydrogenation of but-2-yne-1,4-diol to butane-1,4-diol proceeds via the formation of the intermediate but-2-ene-1,4-diol, which compound is however known to isomerize to .gamma.-hydroxy-butyraldehyde thereby decreasing the yield of the desired alkanediols. Although it is possible in principle to hydrogenate .gamma.-hydroxy-butyraldehyde to butane-1,4-diol, said hydrogenation is considerably more difficult than the hydrogenation of the olefinically unsaturated double bonds. A further problem relates to the preparation of the alkanediol, as described hereinbefore is, that the catalysts which are effective in the hyrogenation of the unsaturated groups, e.g. noble metals and especially palldium (Pd), are frequently also effective isomerization catalyst, i.e. they promote the formation of compounds such as .gamma.-hydroxy-butyraldehyde.
In U.S. Pat. No. 4,438,285, a process is described for the catalytic hydrogenation of but-2-yne-1,4-diol to butane-1,4-diol, which process is conducted at a temperature from 60.degree.-180.degree. C., preferably at a temperature of 110.degree. C., wherein the catalyst comprises ruthenium and palladium in a 4:1 molar ratio, the ruthenium being present in order to catalyze the hydrogenation of any .gamma.-hydroxy-butyraldehyde formed. This process has a disadvantage in that it employs a very expensive catalyst.
Hence it can be concluded that there is need for further improvement in the preparation of alkanediols via a catalytic hydrogenation of the corresponding alkyne- and/or alkenediols.
The problem underlying the present invention is finding a process for the preparation of alkanediols, which does not suffer from one or more of the problems hereinbefore described.
As a result of continuing and extensive research and experimentation, it has been found that it is possible to prepare alkanediols of the before mentioned type, via a catalytic hydrogenation process which can be conducted under mild reaction conditions and employing a less expensive, single noble metal-based catalyst system, i.e. a palladium-based catalyst system.